Brushless direct current (BLDC) motors are used in industries such as appliances, automotive, aerospace, consumer, medical, industrial automation equipment and instrumentation. BLDC motors do not use brushes for commutation, instead, electronic commutation is used. BLDC motors have advantages over brushed DC motors and induction motors such as: better speed versus torque characteristics, high dynamic response, high efficiency, long operating life, longer time intervals between service, substantially noiseless operation, and higher speed ranges. A brushless motor is constructed with a permanent magnet rotor and wire wound stator poles. Electrical energy is converted to mechanical energy by the magnetic attractive forces between the permanent magnet rotor and a rotating magnetic field induced in the wound stator poles. More detailed information on BLDC motors, available at www.microchip.com, may be found in Microchip Application Notes: AN857, entitled “Brushless DC Motor. Control Made Easy” (2002); AN885, entitled “Brushless DC (BLDC) Motor Fundamentals” (2003); AN894, entitled “Motor Control Sensor Feedback Circuits” (2003); AN901, entitled “Using the dsPIC30F for Sensorless BLDC Control” (2004); AN970, entitled “Using the PIC18F2431 for Sensorless BLDC Motor Control” (2005); AN1160, entitled “Sensorless BLDC Control with Back-EMF Filtering Using a Majority Function” (2012); and AN1292, entitled “Sensorless Field Oriented Control (FOC) for a Permanent Magnet Synchronous Motor (PMSM) Using a PLL Estimator and Field Weakening (FW)” (2011); all available at www.microchip.com/motorcontrol, and all are hereby incorporated by reference herein for all purposes.
BLDC motor control provides three things: (1) pulse width modulation (PWM) drive voltages to control the motor speed, (2) a mechanism to commutate the stator of the BLDC motor, and (3) a way to estimate or sense the rotor position of the BLDC motor. Motor speed is directly proportional to applied voltage, so varying the PWM duty cycle linearly from 0% to 100% will result in a linear speed control from 0% to 100% of maximum RPM. PWM may be used to provide a variable voltage to the stator windings of the BLDC motor. The effective voltage provided thereto is proportional to the PWM duty cycle. The inductances of the stator windings act as low pass filters to smooth out the PWM pulses to substantially direct current (DC). When properly commutated, the torque-speed characteristics of a BLDC motor are substantially identical to a DC motor. The PWM generated variable voltage controls the speed of the motor and its available torque.
A three-phase BLDC motor completes an electrical cycle, i.e., 360 electrical degrees of rotation, in typically six steps at 60 electrical degrees per step. Synchronously at every 60 electrical degrees, winding phase current switching is updated (commutation). However, one electrical cycle may not correspond to one mechanical revolution (360 mechanical degrees) of the motor rotor. The number of electrical cycles to be repeated to complete one mechanical revolution depends upon the number of rotor pole pairs.
BLDC motors are not self-commutating and therefore are more complicated to control. BLDC motor control requires knowledge of the motor rotor position and a mechanism to commutate the BLDC motor stator windings. For closed-loop speed control of a BLDC motor there are two additional requirements, measurement of rotational speed and a pulse width modulation (PWM) drive signal to control the motor speed and power therefrom.
To produce motor torque in a synchronous motor, e.g., brushless DC (BLDC) motors or permanent magnet synchronous motors (PMSM), the rotor position needs to be determined. This may be done, for example, with Hall Effect sensors to provide absolute rotor position sensing. However, Hall Effect sensors increase the cost and complexity of a BLDC motor or PMSM. Sensorless BLDC or PMSM control eliminates the need for Hall Effect sensors by monitoring the back electromotive force (BEMF) voltages at each phase (A-B-C) of the motor to determine drive commutation. The drive commutation is synchronized with the motor when the BEMF of the un-driven phase crosses one-half of the motor supply voltage in the middle of the commutation period. This is referred to as “zero-crossing” where the BEMF varies above and below the zero-crossing voltage over each electrical cycle. Zero-crossing can only be detected on the un-driven phase when the drive voltage is being applied to the driven phases. So detecting a change of the BEMF on the un-driven phase from less than to greater than one-half of the motor supply voltage may be used during application of the drive voltage to the two driven phases for a three phase BLDC motor or PMSM.
One of the simplest methods of control for a BLDC motor or PMSM is six step (trapezoidal) commutation. Switching (commutation), e.g., using power transistors, energizes the appropriate two stator windings of a three phase BLDC motor or PMSM depending upon the rotor position. The third winding remains disconnected from the power source. During rotation of the rotor currents, two of the stator winding currents are equal in magnitude and the third unconnected stator winding current is zero (for a WYE connected stator windings). Using this method with a three phase BLDC motor or PMSM there are only six different space vector directions, and as the rotor turns, the current through two of the stator windings (WYE connected stator windings) is electrically switched (commutated) every 60 degrees of electrical rotation so that the current space vector is always within the nearest 30 degrees of the quadrature direction. The current waveform for each winding is therefore a staircase from zero, to positive current, to zero, and then to negative current. This produces a current space vector that approximates smooth rotation as it steps among six distinct directions as the rotor turns. The trapezoidal-current driven BLDC motor or PMSM are popular because of the simplicity of control but suffer from higher torque ripple and lower efficiency than sinusoidal drive.
Sinusoidal commutation drives the three stator windings of the BLDC motor or PMSM with three currents that vary smoothly as the rotor turns. The relative phases of these currents are chosen, e.g., 120 degrees apart, so that they provide for a smoothly rotating current space vector that is always in the quadrature direction with respect to the rotor and has constant magnitude. This eliminates the torque ripple and commutation spikes associated with trapezoidal commutation. However, sinusoidal commutation drive systems are typically more complex and expensive than trapezoidal commutation drive systems.
Such sensor-less determining applications are reliant on back electro-magnetic force (EMF) sensing or estimation generated by the motor. Thus EMF is proportional to speed and the signal degrades as rotational speed is reduced, so ultimately there is a speed whereby the position of the rotor cannot be detected. This limits the low speed performance and prevents the motor from being optimally commutated from rest. Other methods such as a high frequency injection use impedance/inductance variations, respectively, in a synchronous motor to determine the rotor position. Hence there exists a need for an improved rotor position detection method and system for synchronous motors.